Pressure-Impregnated vs. Surface-Applied Fire Retardant Treated Wood: What Specifiers Need to Know
Specifying fire retardant treated wood (FRTW) in commercial construction isn’t just about meeting code — it’s about confidence. Confidence that the material will perform as tested, remain compliant over time, and deliver consistent fire protection throughout the life of the structure.
Yet one of the most common points of confusion for architects, specifiers, and dealers is the difference between pressure-impregnated and surface-applied fire retardant treated wood.
Understanding how these treatments work — and why the method matters — is essential when specifying FRTW for commercial, multifamily, institutional, and industrial projects.
Why the Treatment Method Matters When Specifying FRTW
At a glance, all fire retardant treated wood may appear similar. It’s stamped, labeled, and marketed as fire retardant. But how the fire retardant chemicals are introduced into the wood has a direct impact on:
- Fire performance
- Code compliance
- Long-term reliability
- Consistency across boards
- Suitability for demanding commercial applications
This distinction becomes critical when projects are subject to IBC requirements, third-party inspections, or long-term liability considerations.
Pressure-Impregnated FRTW: Built for Performance and Consistency
Pressure-impregnated fire retardant treated wood is manufactured by placing lumber inside a sealed pressure vessel, where fire retardant chemicals are forced deep into the wood fibers under controlled pressure and vacuum cycles.
Key Characteristics of Pressure-Impregnated FRTW
✔ Full Penetration
The treatment penetrates beyond the surface, reaching the core of the wood. This ensures fire retardant properties are present throughout the entire cross-section, not just on the exterior.
✔ Consistent Fire Performance
Because the chemicals are uniformly distributed, pressure-treated FRTW delivers consistent flame spread and smoke development performance — even if the wood is cut, drilled, or notched during installation.
✔ Proven Code Compliance
Pressure-impregnated FRTW is tested to recognized standards such as ASTM E84 (Steiner Tunnel Test) and is widely accepted for use in commercial and institutional construction.
✔ Long-Term Reliability
The treatment does not rely on surface integrity. Fire resistance remains intact over time, even after handling, cutting, or minor surface wear.
Surface-Applied FRTW: Where Limitations Arise
Surface-applied fire retardant treatments are typically coatings, paints, or spray-on solutions applied to the exterior of the lumber after it is manufactured.
While these products may have a place in limited or decorative applications, they present important limitations for commercial specification.
Key Limitations of Surface-Applied Treatments
✖ Topical Protection
Fire retardant properties exist only on the surface. If the wood is cut, sanded, or damaged, untreated wood is exposed.
✖ Inconsistent Performance
Coverage can vary depending on application method, installer skill, and jobsite conditions — introducing variability that specifiers often seek to avoid.
✖ Durability Concerns
Surface treatments may degrade over time due to abrasion, moisture exposure, or handling, reducing long-term effectiveness.
✖ Restricted Use Cases
Many surface-applied products are not suitable for structural or high-risk commercial environments and may face scrutiny during inspection or plan review.
Longevity and Lifecycle Confidence
When specifying materials for commercial projects, longevity matters just as much as initial performance.
Pressure-impregnated FRTW offers:
- Stable fire retardant properties over the life of the structure
- Predictable performance across large volumes of material
- Reduced risk during inspections and future renovations
- Greater confidence for owners, insurers, and AHJs
Surface-applied products, by contrast, often require ongoing maintenance, reapplication, or verification, which can introduce uncertainty and additional lifecycle costs.
Why Pressure Treatment Is the Standard for Commercial Projects
For commercial, multifamily, and institutional construction, pressure-impregnated FRTW has become the preferred solution — not because it’s marketed louder, but because it performs more reliably under real-world conditions.
Specifiers choose pressure-treated FRTW because it:
- Aligns with building code intent
- Reduces risk in high-occupancy environments
- Provides consistent, verifiable performance
- Simplifies specification language and approvals
- Supports long-term liability protection
In applications where safety, compliance, and accountability are non-negotiable, the treatment process matters as much as the product itself.
Specifying FRTW with Confidence
Confidence comes from clarity. When architects and specifiers understand how fire retardant treated wood is manufactured, tested, and expected to perform, they can make informed decisions that protect both people and projects.
At Biewer Fire Retardant, we believe specifying FRTW should never be ambiguous. Our focus on pressure-impregnated treatment, consistent quality control, and clear documentation is designed to give specifiers the confidence they need — from design through inspection and beyond.
Learn More About Specifying FRTW
Explore additional resources, technical guidance, and specification support at FireTreatedWood.com.